In the conventional fluorescent lamp, an electric glow discharge is created between the positive and negative terminals. The interelectrode space is filled with a gas, commonly a low pressure mercury vapor, that is selected to emit ultraviolet (UV) radiation when the discharge state is energized. This ultraviolet light is used to stimulate a `phosphor` that is coated on the walls of the glass tube.
The word `phosphor` is a term of art in that, contrary to expectations, a `phosphor` need not contain phosphorous. The term is left over from the previous century when these materials typically did contain the element phosphor. The phosphor, when stimulated by UV light or an electron beam, emits visible light or a range of visible light. This visible light is the light commonly used to light offices, homes, to backlight LCD displays and even to light up the display on the CRTs in televisions and computer monitors. The efficiency of the glow discharge creation, the efficiency at which the UV light is created by the glow discharge, and the efficiency at which the phosphor utilizes the UV light to create visible light all act together in a multiplicative manner to create the overall efficiency of the lamp. The electrical energy that is consumed but not utilized to produce visible light is reduced to heat and becomes a thermal burden. This problem is important in office lighting but is critical in the use of fluorescent lamps for backlighting LCD displays. In these displays, the backlight is often the largest energy user, consuming more power than the computer, hard-disk, and the rest of the display.
Phosphors that photoluminesce were originally discovered by the German physicist Johann Wilhelm Ritter in 1801. The photoluminescent materials are used in so many high volume devices today that there has been a large research effort in this field over the last fifty years. This effort has pushed the luminescence properties of these materials to their physical limits.
The emission of visible light (between 400 nm and 690 nm) requires excitation energies which are, at their minimum, given by ##EQU1## where: .lambda. is the wavelength of the specific desired color; c is the speed of light; and h is Planck's constant. The minimum energy required for excitation therefore ranges from 1.8 eV to 3.1 eV.
The excitation energy is transferred to electrons which jump from their ground-state energy level to a level of higher energy. The allowable energy levels are specified by quantum mechanics. The excitation mechanisms are typically the impact of accelerated electrons, positive ions or photons. In a typical color TV, the excitation is created by 30,000 eV electrons. The wavelength of the emitted light is typically independent of varying levels of input energy by these accelerated particles and is usually a function of the phosphor material only. The input particle energy can, however, affect the efficiency of conversion. That is, how many emitted photons are created by the incoming particle.
In fluorescent lamps, a Mercury atom is excited by the impact of an electron having an energy of at least 6.7 eV. This raises one of the two outermost electrons of the Mercury atom from the ground state to a higher, excited state. Upon spontaneous collapse of the electron from this higher state back to the ground state, the energy difference is emitted as UV light having a wavelength of 185 nm, or 254 nm, depending on the particular states involved. A phosphor coating on the lamp tube, such as Calcium Halophosphate with a heavy metal activator such as Antimony or Manganese, is stimulated by this UV photon and, undergoing a similar process, reradiates visible light.
In a solid, such as the phosphor coating, the electronic energy states form bands. In the ground state, most of the carriers are found in the valence band. After excitation by an incoming particle such as an electron or photon, the carriers are elevated in energy into the conduction band. The energy gap between the valance band and the conduction band is equal to the energy of a UV photon. The `activators` are elements or defects that cause energy levels bridging the gap between the valance and conduction bands. When an electron is in one of these states it can return to the ground state by releasing this energy as a photon of visible light. These activation centers can be excited by either direct bombardment by photons or electrons, or by energy transfer from elsewhere in the bulk. The creation of excitons (ion-electron pairs) can occur some distance from the activation site and these excitons can drift to the activation center where the photon emission process can occur. Energy transfer can also take place in the optical domain by the emission of a photon from an initial activation site. This intermediate photon then induces emission of a new photon from a different site.
If when each energetic photon enters a phosphor, it creates one photon of a lower energy, the quantum efficiency is 100%. But its luminescent efficiency is less than 100%. If each incoming photon creates, on average, less than one new photon, then its quantum efficiency is less than 100%. The quantum efficiency of most phosphors is much less than 100%; common Zinc Sulfide phosphors are about 20% efficient and the luminescent efficiency is less than 20%.
The limits in performance in this "classical" phosphor mechanism are that one must pick the phosphor and activator structure to obtain the desired color. This selection is comparable to selection rules in spectroscopy in that the color is not readily adjustable through common industrial techniques such as varying doping concentrations. Instead, different activators or host matrices must be used, along with the attendant differences that go with the selection and materials. The efficiencies obtained are also regrettably low, generally well below 20% energy in/energy out. The engineering results of these problems are poor colors, heat generation and poor battery life.